Time-delay Signature Research Articles

Bidirectional communication with time-delay concealment in a system combining all-optical intensity and electrooptical phase chaos

In this paper, we propose and study theoretically a novel architecture, which combines all-optical intensity chaos and electrooptical-phase chaos generator, allows for bidirectional secure communication. The configuration includes three all-optical chaotic semiconductor lasers and two identical phase-chaos electrooptic delayed oscillators in the optic-fiber link. We investigate the complexity degree of this chaotic time trace generated in this system and the concealment of the time delay signature (TDS), and systematically analyze the influence of the parameter mismatch on synchronization performance , We demonstrate that the system can generate the chaos with high complexity degree through a calculation of the largest Lyapunov exponent (LLE), Lempel–Ziv complexity (LZC), permutation entropy (PE) and fractal box-counting dimension (FBCD); the TDS of an electro-optical delayed oscillator is concealed by calculating the autocorrelation function (ACF) and the delayed mutual information (DMI); the parameter mismatches, such as the TDS, the bias current, the line width enhancement factor (LWEF), etc., have the influence on the synchronization performance; the changes of the bit rate and the modulation index have the influence on Q-factor of the system. Moreover, our numerical simulations also reveal that, under the parameter match condition the delayed chaotic dynamics can be identically synchronized and the synchronization solution is robust. At last, we successfully achieve the simultaneously bidirectional exchange of the messages introduced on the two sides of a link.

Enhancing time-delay suppression in a semiconductor laser with chaotic optical injection via parameter mismatch.

Time-delay signature (TDS) suppression of an external-cavity semiconductor laser (ECSL) is important for chaos-based applications and has been widely studied in the literature. In this paper, the chaotic output of an ECSL is injected into a semiconductor laser and TDS suppression in the regenerated time series is revisited. The focus of the current work is the influence of parameter mismatch on the TDS evolution, which is investigated experimentally and compared systematically to simulations. The experimental results demonstrate that it is much easier to achieve desired TDS suppression in the configuration composed of mismatched laser pairs. Numerical simulations confirm the validity of the experimental results. In the experiments and simulations, the influence of the injection parameters on TDS suppression is also studied and good agreement is obtained.

Investigation of the Dynamical Behavior of a High-Power Laser Diode Subject to Stimulated Brillouin Scattering Optical Feedback

In this work, we propose a new scheme of optical feedback using stimulated Brillouin scattering (SBS). A high-power laser diode emitting at 1450 nm stimulates Brillouin back-scattering in a 4km-long optical fibre and the back-scattered light is injected back into the laser diode for optical feedback. The experimental results with RF spectrum exhibit clearly the Brillouin frequency shift at 11.46 GHz. Although the frequency shift is very large, the laser diode subject to optical feedback using SBS shows abundant dynamics. RF spectrum maps are also established with respect to the laser drive current. The results are compared with the ones for conventional optical feedback and it is clearly shown that the dynamical behaviours for both the configurations are very different. The investigation with autocorrelation functions reveals that the time-delay signature vanishes completely using the SBS feedback scheme unlike the conventional feedback one.

Simultaneous bandwidth-enhanced and time delay signature-suppressed chaos generation in semiconductor laser subject to feedback from parallel coupling ring resonators.

We propose and demonstrate an external-feedback semiconductor laser-based chaos generation scheme supporting simultaneous bandwidth enhancement and excellent time-delay-signature (TDS) suppression, by using parallel-coupling ring resonators (PCRR) as reflector. The characteristics of effective bandwidth and TDS of chaotic signals generated in three indicative PCRR configurations are thoroughly investigated. The numerical results demonstrate that with the nonlinear feedback of PCRR, the TDS of chaos can be efficiently suppressed toward an indistinguishable level, and the bandwidth of chaos in the proposed scheme can also be enhanced, with respect to the conventional optical feedback configuration. The proposed scheme shows a flexible way to generate wideband complex chaos.

Phased-array assisted time-delay signature suppression in the optical chaos generated by an external-cavity semiconductor laser

An external-cavity semiconductor laser (ECSL) outputs high-dimensional chaos, which has potential for various applications, but the unwanted time-delay signature (TDS) might compromise the performance. In this work, a highly integrated and extensible framework of phased-array semiconductor lasers, operating in a steady state when isolated, is employed to postprocess the original chaos generated by an ECSL. Our results demonstrate that such a compact active device enables TDS suppression over wide parameter space. Better performance can be achieved in the proposed scheme compared with the conventional, discrete semiconductor laser subjected to optical chaotic injection. The influence of the injection parameters and the laser separation is studied, which further confirms the feasibility of the proposed scheme for TDS suppression. The phased array can be readily extended to include a large number of elements, and thus the current scheme allows for the generation of multiple independent chaotic signals with no discernible TDS in parallel. This means that the current study may pave the way for parallel random number generation based on optical chaos.

Time-delay signature concealment of chaos and ultrafast decision making in mutually coupled semiconductor lasers with a phase-modulated Sagnac loop.

We propose and experimentally demonstrate the generation of dual-channels chaos with time delay signature (TDS) concealment by introducing a phase-modulated Sagnac loop in mutually coupled semiconductor lasers (MCSL). Furthermore, we demonstrate the utilization of the dual-channels chaos to solve multi-armed bandit (MAB) problem in reinforcement learning. The experimental results agree well with the numerical simulations. For the purpose of comparison, the MCSL with a conventional Sagnac loop is also considered. It is found that the TDS of dual-channels chaotic signals can be better concealed in our proposed system. Besides, the proposed system allows for a better decision making performance in MAB problem. Moreover, compared with the one-channel chaotic system, the proposed dual-channels chaotic system achieves ultrafast decision making in parallel, and thus, is highly valuable for further improving the security of communication systems and the performance of photonic intelligence.

Evaluating entropy rate of laser chaos and shot noise.

Evaluating entropy rate of high-dimensional chaos and shot noise from analog raw signals remains elusive and important in information security. We experimentally present an accurate assessment of entropy rate for physical process randomness. The entropy generation of optical-feedback laser chaos and physical randomness limit from shot noise are quantified and unambiguously discriminated using the growth rate of average permutation entropy value in memory time. The permutation entropy difference of filtered laser chaos with varying embedding delay time is investigated experimentally and theoretically. High-resolution maps of the entropy difference are observed over the range of the injection-feedback parameter space. We also clarify an inverse relationship between the entropy rate and time delay signature of laser chaos over a wide range of parameters. Compared to the original chaos, the time delay signature is suppressed up to 95% with the minimum of 0.015 via frequency-band extractor, and the experiment agrees well with the theory. Our system provides a commendable entropy evaluation and source for physical random number generation.

Time delay signature and bandwidth of chaotic laser output from semiconductor laser

<sec> Semiconductor laser (SL) can output chaotic lasers under external disturbances such as optical injection or optical feedback, and the bandwidth can reach up to GHz magnitude. External-cavity feedback semiconductor lasers can output high-dimensional chaotic lasers and are considered to be better sources of chaotic entropy. However, due to external cavity feedback and other effects, it will give rise to obvious external cavity time delay signature (TDS) in the output chaotic laser, which restricts the application of chaotic lasers. On the other hand, the bandwidth of chaotic laser determines the transmission rate of confidential communication, and therefore TDS and bandwidth are two important parameters that will affect chaotic laser’s applications. Therefore, it is significant to take appropriate measures to suppress the TDS and increase the bandwidth of chaotic laser output by semiconductor laser. </sec><sec> In this paper the output laser from a semiconductor laser with single optical feedback is partially injected to another semiconductor laser with double filtered optical feedback. Thus they form a semiconductor laser system with external optical injection and double filtered optical feedback, i.e. a master-slave laser system which is used to suppress the TDS of chaotic laser and investigate its bandwidth. We numerically investigate the influences of external light injection coefficient, feedback intensity, pumping factor, and filter bandwidth on TDS. Then the suppression effects of this system on TDS are analyzed and compared with those of semiconductor laser system with external optical injection and single optical feedback, those of semiconductor laser system with external optical injection and double optical feedback, those of semiconductor laser system with external optical injection and single filtered optical feedback, and those of semiconductor laser system with double filtered optical feedback. The results show that the proposed scheme in this paper has the best suppression effect on TDS. Then the bandwidth of the chaotic laser output from the system is investigated under the condition of parameters of effectively suppressing TDS. The results show that the system proposed in this paper can increase the bandwidth of the system output chaotic laser by properly selecting the parametric values, and the maximum bandwidth value of the obtained chaotic laser is about 8.8 GHz. The above investigations indicate the effectiveness of the proposed scheme. The results of this investigation are significant for the application of chaotic lasers. </sec>

Suppression of Cavity Time-Delay Signature Using Noise-Phase-Modulated Feedback

We propose reconfiguration of the conventional feedback scheme to suppress the external-cavity time-delay signature (TDS) by adding noise phase modulation in the feedback of semiconductor laser. Noise-phase-modulated feedback introduces broad band noise frequencies into the feedback light and enables the suppression of the TDS. In this work, by means of simulations, the effect on TDS suppression of phase modulation (PM) index is explored. In addition, the influence of noise bandwidth variation is investigated. Using the auto-correlation function to quantity the TDS, we find that, for a large range of operating parameters, the TDS is significantly suppressed to the noise level and even submerged into the base noise. It is shown that suppression TDS is achievable over a wide operating parameter provided the noise generator bandwidth is of order 10 GHz and the PM index is greater than about 3. The proposed configuration will have widespread applications in contexts where suppression of the TDS is required.

Neutrino Echoes from Multimessenger Transient Sources.

The detection of the high-energy neutrino event, IceCube-170922A, demonstrated that multimessenger particle astrophysics triggered by neutrino alerts is feasible. We consider time delay signatures caused by secret neutrino interactions with the cosmic neutrino background and dark matter and suggest that these can be used as a novel probe of neutrino interactions beyond the standard model (BSM). The tests with BSM-induced neutrino echoes are distinct from existing constraints from the spectral modification and will be enabled by multimessenger observations of bright neutrino transients with future experiments such as IceCube-Gen2, KM3Net, and Hyper-Kamiokande. The constraints are complementary to those from accelerator and laboratory experiments and powerful for testing various particle models that explain tensions prevailing in the cosmological data.

Time-delay signature characteristics of the chaotic output from an optoelectronic oscillator by introducing an optical feedback.

In this work, via autocorrelation function (ACF) and permutation entropy (PE) methods, we numerically investigate the time-delay signature (TDS) characteristics of the chaotic signal output from an optoelectronic oscillator (OEO) after introducing an extra optical feedback loop. The results demonstrate that, for such a chaotic system, both the optoelectronic feedback with a delay time of T1 and the optical feedback with a delay time of T2 contribute to the TDS of generated chaos. The TDS of the chaotic signal should be evaluated within a large time window including T1 and T2 by the strongest peak in the ACF curve of the chaotic signal, and the strongest peak may locate at near T1 or T2. Through mapping the evolution of the TDS in the parameter space of the optical feedback strength and time, certain optimized parameter regions for achieving a chaotic signal with a relatively weak TDS can be determined.

Security-enhanced chaotic communications with optical temporal encryption based on phase modulation and phase-to-intensity conversion

We propose and numerically demonstrate a security-enhanced chaotic communication system by introducing optical temporal encryption (OTE) into the modulated chaotic carrier (chaos + message). In the proposed scheme, the message is firstly embedded into the original chaotic carrier generated by a conventional external-cavity semiconductor laser (ECSL), and before being transmitted to the receiver end, the modulated chaotic carrier propagates through an OTE module that consists of one phase modulator driven by a secret sinusoidal signal and one dispersive component. Our numerical results indicate that, as a direct result of the spectral expansion effect of the sinusoidal phase modulation and the phase-to-intensity conversion effect of the dispersive component, the original chaotic carrier can be encrypted as an uncorrelated chaotic signal with a flat spectrum and an efficiently-suppressed time delay signature, this greatly enhances the privacy of the modulated chaotic carrier. Moreover, comparing with the conventional ECSL-based chaotic communication systems without OTE, the proposed scheme not only shows significantly higher security against attacks including direct linear filtering and synchronization utilization, but also provides additional physical key space to further enhance the system security. In addition, by making use of the transmission dispersion for decryption, the proposed encryption scheme supports dispersion-compensation-free secure fiber communication, and it also supports centralized encryption/decryption in wavelength division multiplexing secure chaotic communication systems. The proposed scheme explores a novel encryption method for implementation in high-security chaotic communication systems.

Real-Time 2.5-Gb/s Correlated Random Bit Generation Using Synchronized Chaos Induced by a Common Laser With Dispersive Feedback

We experimentally demonstrate high-speed correlated random bit generation in real time using synchronized chaotic lasers commonly driven by a laser with dispersive feedback. The dispersive feedback from a chirped fiber Bragg grating induces frequency-dependent feedback delay and thus no longer causes time-delay signature, and resultantly ensures the signal randomness and security of chaotic laser. Driven by the time-delay signature-free chaotic signal, the two response lasers are routed into chaotic states and establish a synchronization with correlation beyond 0.97 while they maintain a low correlation level with the drive signal. Through quantizing the synchronized laser chaos with a one-bit differential comparator, real-time 2.5-Gb/s correlated random bits with verified randomness are experimentally obtained with a bit error ratio of 0.07. Combining with a robust sampling method, the BER could be further decreased to 1×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> corresponding to an effective generation rate of 1.7 Gb/s. Bit error analysis indicates that the bit error ratio between the responses is lower than that between the drive and responses over a wide parameter region due to the synchronization superiority of the responses over the drive.

High-speed optical secure communication with an external noise source and an internal time-delayed feedback loop

We propose and experimentally demonstrate a novel physical layer encryption scheme for high-speed optical communication. A 10 Gb/s on-off keying signal is secretly transmitted over 100 km standard single-mode fiber. The intensity-modulated message is secured by the encryption mechanism, which is composed of an external noise source and an internal time-delayed feedback loop. The external noise serves as an entropy source with sufficient randomness. The feedback loop structure in the transmitter introduces a time-domain encryption key space, and a corresponding open-loop configuration at the receiver side is used for synchronization and decryption. Experiment results show the effectiveness of the proposed scheme. For a legitimate terminal, bit error rate below 10−8 can be obtained. Decryption degradations with the mismatch of different hardware parameters are researched. The time delay in the feedback loop provides a sensitive encryption key. For other hardware parameters, the system is robust enough for synchronization. Meanwhile, the time-delay signature of the loop is able to be well concealed by the external noise. Moreover, the proposed scheme can support density wavelength division multiplexing transmission with a relatively simple structure. This work also provides a new concept to establish optical secure communication by combining a time-delayed feedback chaotic system and random noise.

Isochronous cluster synchronization in delay-coupled VCSEL networks subjected to variable-polarization optical injection with time delay signature suppression.

The isochronous cluster synchronization with time delay (TD) signature suppression in delay-coupled vertical-cavity surface-emitting laser (VCSEL) networks subject to variable-polarization optical injection (VPOI) is theoretically and numerically studied. Based on the inherent symmetries of network topology, parameter spaces for stable cluster synchronization are presented, and zero-lag synchronization are achieved for VCSELs in same clusters. Additionally, the TD signature reduction for the dynamics of VCSELs in the stable clusters are systematically discussed. It is shown that both moderate polarizer angle and frequency detuning between different clusters have strengthen the effect of TD signature suppression. Moreover, the isochronous cluster synchronization with TD signature concealment is also verified in another VPOI-VCSEL network with different topology, indicating the generality of proposed results. Our results shed a new light on the research of chaos synchronization and chaos-based secure communications in VCSEL networks.

Permutation entropy analysis of chaotic semiconductor laser with chirped FBG feedback

We numerically analyze the chaos complexity of a semiconductor laser with chirped fiber Bragg grating (CFBG) feedback by utilizing the permutation entropy (PE). Due to dispersion-induced nonlinearity of chirped grating, the time delay signature (TDS) can be suppressed or erased and the complexity enhancement is expected. The effects of laser parameters on PE as well as the TDS are studied in detail. The results show that for CFBG feedback, the TDS can be suppressed in the range of parameters where linewidth enhancement factor exceeds 3 and gain saturation coefficient is lower than 4×10−4μm3, and the complexity of chaotic signals is enhanced. As the feedback strength and bias current increase, the PE value under CFBG feedback increases linearly to a maximum of 0.994 and remains stable. Compared with mirror feedback, CFBG feedback can achieve increased complexity over a wider range of parameters. In addition, when the dispersion value of the chirped grating exceeds 2000 ps/nm, the frequency detuning range of -15 GHz to 10 GHz is beneficial to obtain chaotic signals with higher PE values.

Long-Range and High-Precision Fault Measurement Based on Hybrid Integrated Chaotic Laser

We propose a long-range, high-precision optical time domain reflectometry (OTDR) technology based on the hybrid integrated short-external-cavity chaotic semiconductor laser (SCSL) which has the advantages of small volume, simple process, low cost, mass production, stable performance, and high yield. The chaotic signal generated by the hybrid integrated SCSL exhibits a certain extent of oscillations and fluctuates randomly in time series. It can be seen from the auto-correlation curve that the generated chaotic signal has no time-delay signature and the theoretical spatial resolution is 1 cm according to the full width of half maximum. In this letter, not only the measurement of multiple single reflection events but also the measurement of double-reflection events was realized, and the spatial resolutions at different distances were verified. Finally, the spatial resolution and range-independent breakpoints measurement has been realized, and the detection range of 76.54 km and the spatial resolution of 1 cm (7.7 million resolved points) have been experimentally demonstrated.

Wideband Time Delay Signature-Suppressed Chaos Generation Using Self-Phase-Modulated Feedback Semiconductor Laser Cascaded With Dispersive Component

We propose a wideband chaos generation scheme that shows significant bandwidth enhancement and excellent time delay signature (TDS) suppression, by introducing self-phase modulation in the feedback loop of external-cavity semiconductor laser (ECSL) and passing the ECSL-generated chaos through a dispersive component. Proof-of-concept experiment and thorough complementary numerical simulations are carried out to study the efficient bandwidth and TDS characteristics of the generated chaos. The results demonstrate that with the joint contributions of self-phase modulation and dispersion, the efficient bandwidth of chaos can be enhanced by several times and the spectrum gets much flatter. Simultaneously, the TDS can be completely suppressed, in a wide operation parameter range. The proposed wideband chaos generation scheme shows great potential for the applications of fast random bit generation, high-resolution chaotic radar, and high-speed chaos communication.

2.24-Tb/s Physical Random Bit Generation With Minimal Post-Processing Based on Chaotic Semiconductor Lasers Network

Multi-channel physical random bit generator (RBG) based on a chaotic semiconductor lasers (SLs) network is proposed and experimentally demonstrated. In experiments, a small ring network consisting of three SLs mutually coupled with heterogeneous delays is constructed to obtain chaotic temporal waveforms. The results show that the time-delay signature of chaotic intensities in all the three SLs can be well concealed over wide range of parameters. Through linear combination of the three chaotic outputs, seven channels of chaotic entropy sources can be obtained. Furthermore, by introducing a minimal post-processing technique and directly extracting four least significant bits at 80-GS/s sampling rate, seven channel random bit sequences with verified randomness can be simultaneously achieved, and the total bit rate is 2.24 Tb/s. The proposed RBGs enabling such high generation rate have potential applications in the one-time pad encryption for the high-speed optical communication networks.

Wideband complex-enhanced chaos generation using a semiconductor laser subject to delay-interfered self-phase-modulated feedback.

A wideband complexity-enhanced chaos generation scheme is proposed by using a semiconductor laser subject to delay-interfered self-phase-modulated optical feedback. The influences of feedback strength, phase modulation index, and interference delay on the effective bandwidth and time-delay-signature (TDS) characteristics of the proposed scheme-generated chaos are extensively investigated both experimentally and numerically. The results demonstrate that with the joint effects of phase modulation-induced spectrum expansion and nonlinear filtering of delayed interference, wideband chaos with flat spectrum and excellent TDS suppression characteristics can be generated over a wide dynamic operation range. In comparisons with the relevant chaos generation schemes under conventional optical feedback, individual self-phase modulated optical feedback, and delay-interfered optical feedback, the proposed scheme cannot only significantly enhance the effective bandwidth of chaos but also considerably enhance the complexity of chaos by suppressing the TDS toward an indistinguishable level close to 0.